Observed behaviour:

$ linux.uml
Core dump limits :
        soft - 0
        hard - NONE
Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...OK
Checking advanced syscall emulation patch for ptrace...OK
Checking environment variables for a tempdir...none found
Checking if /dev/shm is on tmpfs...OK
Checking PROT_EXEC mmap in /dev/shm...Operation not permitted
/dev/shm must be not mounted noexec
$

Expected behaviour: it works, without me having to reconfigure
/dev/shm to a less-secure configuration.

Workaround:

bwrap --dev-bind / / --tmpfs /dev/shm linux.uml ...

FYI I first encountered this with uml from buster,
  4.19-1um-1+deb10u1+b1
but the bug is in sid too.